In ancient India, saltpetre was used for fireworks; it is actually:

The Correct Answer is: (a) Potassium Nitrate

Explanation:
Saltpetre, also known as niter, is the common name for Potassium Nitrate (KNO₃). It is a white crystalline substance that was historically extracted from soil enriched by organic material.

When mixed with charcoal and sulfur, it forms black powder, one of the earliest forms of gunpowder. India was among the earliest civilizations to use such materials for

military, mining, and ceremonial purposes, including firework displays during festivals.

Information Booster:
• Potassium nitrate contributes oxygen to a combustion reaction, enabling rapid burning.​
• India was a major producer and exporter of saltpetre during the medieval and colonial periods.
• It is also used in preserving meat, fertilizers, and rocket propellants.

Additional Information:
• Sodium Nitrate –  known as Chile saltpetre, used in explosives and fertilizers.
• Calcium Nitrate – Used in fertilizers and wastewater treatment.